1. Field of the Invention
The present invention relates to a dielectric ceramic composition. More particularly, it relates to a ceramic composition useful as a dielectric material for small-sized, high capacitance multilayer tip capacitors.
2. Description of the Prior Art
So far, dielectric ceramic compositions mainly comprising BaTiO.sub.3 have widely been used as a dielectric material for multilayer ceramic capacitors because of their high relative dielectric constant. However, such BaTiO.sub.3 compositions have some disadvantages awaiting a solution. One of the disadvantages is that they have a high sintering temperature of not less than 1300.degree. C. Thus, when producing multilayer capacitors, it is required to use noble metals such as Pt and Pd as a material for internal electrodes, resulting in high cost of production.
To this end, it has been proposed recently in a few patents to use a dielectric ceramic composition mainly comprising PbTiO.sub.3 as a dielectric material for multilayer ceramic capacitors. For example, Japanese patent laying-open No. 57-27974 discloses a composition of a system Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3, and Japanese patent laying-open No. 62-115608 discloses a composition of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --Pb(Zn.sub.1/3 W.sub.2/3)O.sub.3 --PbTiO.sub.3. Also, it has been proposed to use a dielectric ceramic composition of a system Pb(Fe.sub.2/3 W.sub..DELTA.)O.sub.3 --Pb(Fe.sub.1/2Nb.sub.1/2)O.sub.3 --PbTiO.sub.3.
Such PbTiO.sub.3 -based ceramic compositions have a high relative dielectric constant ranging from 10000 to 15000 and a low sintering temperature of about 1000.degree. C., thus making it possible to use relatively low-cost silver alloys such as Ag-Pd alloys as an internal electrode material for multi-layer ceramic capacitors.
However, these ceramic compositions are poor in resistance to plating because of a low redox potential of Pb contained therein, thus making it difficult to use them as a dielectric material for surface-mounting capacitive elements. Because, when producing capacitors for surface mounting, it is required to form an electrode layer of Ni, Sn or Sn-Pb alloy by electro-plating. During electroplating on capacitor units or chips, reduction of lead (Pb) takes place on the surface of the capacitor unit to give conductivity to the chip surface, resulting in short-circuiting of the electrodes due to development of a plated metal. Also, it causes failure in appearance and lowers the Q factor and insulating resistance of the product.